How to avoid successive deallocations/allocations in C++?

Question

Consider the following code:

class A
{
    B* b; // an A object owns a B object

    A() : b(NULL) { } // we don\'t know what b will be when constructing A

             


        

Answer 1

Simply reserve the memory required for b (via a pool or by hand) and reuse it each time you delete/new instead of reallocating each time.

Example :

class A
{
    B* b; // an A object owns a B object
    bool initialized;
public:
    A() : b( malloc( sizeof(B) ) ), initialized(false) { } // We reserve memory for b
    ~A() { if(initialized) destroy(); free(b); } // release memory only once we don't use it anymore

    void calledVeryOften(…)
    {
        if (initialized)
            destroy();

        create();
    }

 private:

    void destroy() { b->~B(); initialized = false; } // hand call to the destructor
    void create( param1, param2, param3, param4 )
    {
        b = new (b) B( param1, param2, param3, param4 ); // in place new : only construct, don't allocate but use the memory that the provided pointer point to
        initialized = true;
    }

};

In some cases a Pool or ObjectPool could be a better implementation of the same idea.

The construction/destruction cost will then only be dependante on the constructor and destructor of the B class.